The application of functional thin film coatings to substrates is old in the art. Typically, such thin film coatings are applied to plastics to enhance their utility in electrical or electronic environments. The need for such coatings arises from the permeability of conventional materials to interference signals. For example, computer housings, keyboard housings, CATV housings and bezels, electronic radio housings and bezels, and digital telephone housings are frequently made of plastic or polymeric materials. Electronic or electrical equipment contained within unshielded plastic or polymeric housings are all susceptible to the effects of electromagnetic interference (EMI), radio frequency interference (RFI), and electrostatic discharge (ESD).
To protect against the adverse effects of such interference or discharge, the inner surfaces of such housings are typically coated with a thin metal film. Various methods are usable for the deposition of such films. One is conventional vacuum metallizing, which generally uses a lacquer base coat on the plastic substrate to enhance adhesion of the film. Another is functional thin film coating, which is the generic name for the batch process that is the basis of the present invention.
In functional thin film coating, the substrate or part to be coated is generally placed in a fixture mask having individual cells whose shape largely conform to that of the part itself. The substrate is placed within a cell, which covers or shields those portions of the substrate not intended to receive the film coating.
The fixture mask is then placed on a planetary work holding assembly, which is in turn placed in a vacuum chamber. When the system operator has placed the vacuum chamber at a suitable plating vacuum, the vacuum pumping system is shuttered and argon gas is bled into the chamber until a predetermined, designated pressure is attained. The argon is then electrically excited with an appropriate voltage and current, and the excitation causes a cloud of free electrons, or plasma, to form within the vacuum chamber. Upon contact with this plasma, an electrical charge is imparted to the exposed or non-shielded portions of the substrate. In fact, the surface is prepared by a process called glow-discharge cleaning. In this specification, it should be understood that the terms "imparting an electrical charge" and "surface preparation," or their respective equivalents, shall be used interchangeably.
In glow-discharge cleaning, impurities are removed from the surface and other beneficial changes are effected. These beneficial changes on the substrate surface occur as a result of one or more of the following steps: (1) straightforward heating due to impingement of charged particles and their recombination; (2) impurity desorption through electron bombardment; (3) impurity desorption resulting from low-energy ion or neutral-particle bombardment; (4) volatilization of organic residues by chemical reaction with dissociated oxygen; (5) modification of glass surfaces through the addition of oxygen; and (6) enhanced nucleation during subsequent film deposition. From page 6-41 of "Handbook of Thin Film Technology," Maissel and Glang, published by McGraw-Hill.
After a short plasma exposure, the argon gas supply is stopped and the vacuum pumps are unshuttered. Typically, a plurality of tungsten or other suitable metallic, conducting electrodes are provided within the vacuum chamber. Each of the electrodes serves to support one or more small clips or canes fabricated from the desired metal. These clips act as the source of a "plating charge." After the argon gas supply has been stopped and the desired plating vacuum has been attained in the vacuum chamber, an electrical current is supplied to the tungsten electrodes. In time, the current causes the small metal clips to evaporate. The resultant evaporated metal migrates toward and is deposited upon the prepared or charged, exposed surface of the substrate. Upon deposition, the metal cools to form a thin metal film coating on the substrate.
Finally, chamber vacuum is broken, the planetary work holding assembly is removed, and a new batch process may be initiated.
Among the metals that have been applied by functional thin film coating are aluminum, aluminum alloyed with other metals, and stainless steel. More than one layer of thin film may be deposited on a substrate. Such multiple-layer thin films are useful in hostile environments. For example, copper thin films may be most suitable in environments where the substrate is subject to the deleterious effects of abrasion, chemical attack, or moisture. The copper, however, may be undesirably corroded adjacent the porous surfaces of polymeric substrates. Thus, an insulating base, thin film of stainless steel is applied directly to the substrate, and the copper thin film is applied over this stainless steel base film layer.
One method for the deposition of a chromium or stainless steel composite layer is described in U.S. Letters Pat. No. 4,544,571, issued to Walter J. Miller on Oct. 1, 1985, and entitled "Method of Manufacture of EMI/RFI Vapor Deposited Composite Shielding Panel."